Teachers' notes

Geological time

For many, the concept of time is difficult to grasp. Yet even in
Key Stages 1 and 2, children are expected to understand long periods
of time: the activities of the Egyptians 4 000 years ago, the Romans
2 000 years ago or the Vikings 1 000 years ago. If one thousand years
is a long time, how can a child comprehend one thousand million years?
But not only is it possible, it is intellectually stimulating. Geological
time means large numbers and it is necessary to break down these
vast periods of time into more manageable pieces. Scaling is a useful
tool.

There are a number of 'models' that have been used to scale the
passing of the 4 600 million years of the Earth's geological history.
Compressing geological time into, for example, a 460-page book, the
twenty four hours of the day or even to a single hour have been used.
These are misleading, however, as they gives the impression that,
with the appearance of humans 'a few seconds before midnight', geological
time came to an end and that our species is the ultimate life form
at the end of the long evolutionary process. This is far from the
truth. If 'survival of the fittest' is the key to evolution, we have
a lot to learn from blue-green cyanobacteria!

The Geological Timeline

It has been estimated that the solar system is about half way through
its life, so the model presented here scales geological time to that
of a middle-aged person. We consider the Earth not as a planet 4 600
million years old, but as a person 46 years old today. Of course
to an eight or 12 year old child, it is difficult to imagine that
anybody could be as old as 46, but discussion about the age of relatives
(parents and grandparents – and teachers?) makes this more understandable.
For a child, the same mental agility is necessary to come to terms
with 46 years, as for 460 years (the Tudor period), 4 600 years (Egyptians
were about to construct the pyramids) and 4 600 million years (the
age of the Earth).

The Geological Timeline introduces the question— what of the future?
Our middle-aged Earth is part way through its life. It is interesting
to debate what the world's environment might be in another million
or 4 000 million years. Will humans still exist? What will be the
effect of humans on biodiversity? What organisms might evolve? What
might the atmosphere be composed of? How will it all end?

Classroom activities

A timeline is the ideal way to comprehend the passage of geological
time and to demonstrate how life, environment and geography has changed
throughout Earth's history. There are several ways that this can
be achieved in school.

A piece of wall-paper 4.6 metres long could be stretched around
a room, divided up into forty-six rectangles.

A 4.6 m length of rope, like a washing line, can be stretched
across a playground with a peg every 10 cm .

An interesting project is to draw the timeline by computer.

Whatever method is appropriate, subdivision into 46 units is useful
as these give the idea of the 46 'birthdays' (each birthday represents
100 million years of geological time). It is helpful if the last
division (representing the last year) is an elongate rectangle divided
into 12 'months'— a lot happened during that last 'year'. The children's
work can be attached to the timeline—written work, photographs or
art work.

The key geological events in the history of life are shown in the
Geological Timeline and further details are shown in the table below.
It should be noted that some of these events did not fall exactly
on the Earth's 'birthdays', but they have been placed next to the
nearest one. So the appearance of the dinosaurs 225 million years
ago, for example, has been placed at the 44th birthday (so within
25 million years). Of course absolute ages are approximate anyway
and the degree of error and uncertainty increases with age. Taking
the Carboniferous as an example, its lower boundary been variously
dated between 367 and 353 million years ago and the top between 280
and 301, and is, therefore, between 52 and 87 million years in duration.

Cross-curricular activities

Science—what is life; relationship between organisms
and the environment they live in; changes in the environment; Earth
studies

Art—depiction of life in the past

IT— construct a time line on a PC. Student's artwork
can be scanned and attached to their time line (or digitally based
artwork packages can be used). Pictures of fossils, rocks, minerals,
maps, etc, perhaps taken from the Web or from clip-art packages
can be used. Hyperlinks can be used to link the time line to additional
information (e.g. students' written work or art work).

Theology—this is a difficult subject, but here we present
the scientific view of geological time and the evolution of life.
How does this compare with religious beliefs?

Further reading

A number of scientific palaeontology books are available, providing
data that can be used on a timeline. The majority are for advanced
students, but a brief outline of the fossil record, including a short
reference list, is published by the British Geological Survey:
Rigby, S 1997. Fossils, the story of life. 64pp [British Geological
Survey, Keyworth]

The Fossil Focus series is also published by the British Geological
Survey. These laminated A3 cards colourfully explain the anatomy,
distribution and environmental requirements of a number of fossil
groups. A list can be found in the BGS catalogue.

The timeline in 'birthdays'

Approximate age of the Earth (millions of years)

Approximate time before present (millions of years)

Notes on key events along the timeline

0

0

4600

Earth formed from a dust cloud with the sun in centre. When
Earth was about 80% of its present size, it crashed with another
planetoid. Debris around the Earth fused together to form
the moon.

1

100

4 500

Earth's core formed when dense metals sank to the centre.
Eventually the stony crust cooled and solidified. Little is known
about Earth (no crustal rocks survive).

2

200

4 400

Oldest known minerals to form on Earth are zircon crystals
in Australia. Inclusions in the crystals said to indicate oceans had formed by
this time, although this is controversial.

5

500

4 100

End of the Hadean (the name of the essentially unknown phase
of Earth's history, not represented by crustal rocks).

Akilia Gneiss (3 850 Ma) said to have carbon traces of life,
but this is controversial.

9

900

3 700

Banded Ironstone Formations (BIFs) are considered to have
been created by bacteria. Oxygen created by bacteria caused ferrous
iron in the ocean water to oxidise and precipitate as a red layer
of iron on the sea floor. At times when oxygen was not being
created, grey cherts were precipitated instead. These layers
built up alternately to form BIFs. BIFs provide the earliest
signs of photosynthesis. They began to form about 3 700 Ma, but
no fossils are known. However, photosynthesising bacteria must
have evolved from non-photosynthesising ancestors, which in turn
evolved via non-biological evolution.

11

1 100

3 500

The Apex Chert (western Australia), 3 465 Ma old, was once
believed to contain the earliest fossils, but this is now considered
unlikely. However, in the Pilbara region, NW Australia, silica-rich
rocks dating to about 3 500 Ma contain tubes about 40 microns
long and thinner than a human hair. Although some may have formed
inorganically, there are some geologists who believe that they
were formed by rock-eating bacteria. This is still controversial.

16

1 600

3 000

Stromatolites formed by blue-green cyanobacteria, microscopic,
single celled, photosynthesising organisms. The organic mats precipitated
calcite and trapped sediment particles into the layer. Blue-green bacteria
are still making stromatolitic domes in Shark Bay (Australia) 3 000
million years later.

25

2 500

2 100

Oxygen was a waste product produced by bacteria and would
have been poisonous to these early life forms. Initially the
oxygen was chemically trapped in the rocks, e.g. BIFs and limestones, but
eventually there was too much to store in this way and it escaped
into the atmosphere. Terrestrial 'Red Beds' were created 2 100
Ma, by the oxidisation of iron. Eventually free oxygen began
to accumulate in the atmosphere.

31

3 100

1 500

Small amounts of oxygen in the atmosphere. The first eukaryotes
appear, the basic cell type that almost every living thing on
Earth is made of—protista, fungus, plant, animal kingdoms (only
bacteria, Kingdom Monera, have the simpler prokaryotic cell).
Eukaryotes require oxygen for their metabolism. Sexual reproduction
is said to have evolved at about this time. Rocks 1 000 Ma old
show an increase in diversity of these early eukaryotes, protista.

39

3 900

700

Geneticists have suggested animal life began c. 1 000 Ma ago,
but there is no evidence for this in the geological record. Choanoflagellates
are protistids with genetic material also found in animals and
it has been suggested that the animal kingdom evolved from something
similar. The earliest trace fossils in Australia (made by the activities of presumably soft-bodied animals) and Africa are
about 700 Ma old.

40

4 000

600

The first multicelled animal fossils including the 'sea-pen' Charnia,
worms, sea urchin-like creatures and jelly fish, are a little
over 600 Ma old. Fecal pellets discovered in 600 million year
old rocks in Scotland must have been left by an animal with a
gut.

41

4 100

500

Animals with hard parts (shells and skeletons) e.g. trilobites
and molluscs evolved 545 million years ago (at the beginning of the
Cambrian). Soon afterwards, all kinds of organisms with hard
parts began to evolve, including corals, crinoids, brachiopods,
nautiloids, graptolites and microscopic species too (e.g. foraminifera).
The earliest fish evolved in the early Cambrian. The first fish
with calcareous back bones (rather than cartilaginous notocords)
evolved a little later. Comparison can be made to other vertebrates
including ourselves.

42

4 200

400

Sufficient ozone in the atmosphere allowed plants to evolve
from algae and colonise the land. Invasion of land by plants
began with the evolution of non-vascular bryophytes in the Mid
Ordovician, about 450 Ma ago. Cooksonia, the first vascular
plant, evolved in the late Silurian, c. 420 Ma ago. Soon afterwards
animals followed the plants.

43

4 300

300

Animal life has been found on the marshy land associated with
early plant fossils. Worms, snails and, by the late Devonian
(about 350 Ma), the first amphibians (tetrapods) left the aquatic
realm. Amphibians rapidly evolved into lizards. Lizards had developed
a water proof egg that did not have to be laid in water. They
did not have a need to stay close to bodies of water and keep
wet. The first tropical rain forests evolved (the coal forests)
and began to spread about 320 Ma ago.

44

4 400

200

Lizards evolved into dinosaurs 225 Ma ago. There are a number
of differences, but the most obvious is in the construction of
the hip so that dinosaurs were able to stand with straight legs
beneath their body. Some were bipedal. 'Mammal-like reptiles'
evolved into the first mammals—shrew-like insectivores about
210 Ma ago.

45

4 500

100

Archaeopteryx, the first bird, evolved from feathered
theropod dinosaurs about 140 Ma ago. Soon afterwards (c.130 Ma
ago) flowering plants evolved—Archaefructus was the
earliest angiosperm (it had carpels but no flower), but soon
afterwards species related to magnolia appeared (oldest fossil
flower).

8 months ago

4 535

65

Mass extinction of 65 to 70% of all species, including all
the ammonites, belemnites, flying reptiles and dinosaurs (although
birds, the last of the evolutionary line of the dinosaurs continued
to thrive).

7 months ago

4 550

50–60

After extinction of the dinosaurs, very rapid mammalian evolutionary
radiation, especially in the Eocene (about 40–55 million years
Ma), occupying land, sea and air.

4 months ago

4 565

35Ma

The first primates, related to lemurs, evolved late in the
Cretaceous, just before the mass extinction and the disappearance
of dinosaurs, ammonites etc. Monkeys evolved about 35 Ma ago
and started to evolve rapidly: the dryopithecines appeared about
25 Ma and the first apes 17 Ma. .

2 months ago

4 585

15

Grass evolved. This is, perhaps, the most important flowering
plant so far as humans are concerned as it provides us with wheat,
barley, maize, rice, etc. The first grasslands evolved during
a prolonged phase of climatic cooling. A number of animals took advantage
of the expanding grasslands, including some of the primates that
live on the ground rather than the trees.

About 3 weeks ago

Almost 4 600

c. 5-6

The hominid Australopithecus evolved ('hominid' and
'human' should not be confused; Australopithecus was
not human). There have been several species. Recently a skull
c. 7 Ma old was discovered that has been suggested to be the
earliest hominid but some believe it to be the skull of an ape. Australopithecus
afarensis ('Lucy') evolved about 5 Ma ago and the last species
of Australopithecus boisei ('Nutcracker Man'), lived
from 2.3 to 1.4 Ma ago.

About 7–8 days ago

2–2.5

The first species of human evolve in Africa—Homo habilis.
Fossils of their brain case shows that the speech centre is only
just beginning to develop.

c. 6–7 days ago

2–1.6

Homo erectus is considered by some to be two species-H.
ergaster and H. erectus. They evolved in Africa
about 1.6 Ma ago. Homo erectus was the first human
species to migrate across Europe and Asia.

c. 5 days ago until 'last night'

1.3

Ice ages start, but this was a period of very variable climate–Britain
was sometimes buried beneath about 1 km thick ice caps, sometimes
tundra developed, sometimes it was warmer than today. During
warm periods, lions, hippo and rhino lived in Britain, but when
the tundra developed, mammoth, wolves and giant elk lived here.
The last glaciation in Britain ended about 10 000 years ago.

c. 2 days ago

0.5

Homo heidelbergensis evolved. The earliest fossils
are c. 500 000 years old. 'Heidelberg Man' is also known as 'Swanscombe
Man' and 'Boxgrove Man' in Britain.

The timeline in 'birthdays'

Approximate age of the Earth (millions of years)

Approximate time before present

Notes on key events along the timeline

46

c.13 hours ago

4 600

150 000 years ago

Homo neanderthalensis evolved and spread throughout
Europe during the 'Ice Ages' - fossils are found particularly
in Mediterranean countries, but also Germany (the type area)
and they also reached Britain. It possibly evolved from Homo
heidelbergensis.

c. 12 hours ago

130 000 years ago

Modern humans (Homo sapiens) evolved in Africa about
130 000 years ago, perhaps from 'Rhodesia Man', Homo rhodesiensis,
but the evolution of Homo is controversial and there
are a number of different evolutionary theories. The idea that
we evolved from Homo neanderthalensis is flawed.

c.3.5 hours ago

c. 35 000 years ago

Humans left Africa c.35 000 years ago to spread over Europe
and Asia. Neanderthals became extinct about 30 000years ago.

c.1 hour ago

c.11 000 years ago

Man became a farmer.

c.1 minute ago

250 years ago

The Industrial Revolution. During the last 60 seconds we have
pollution, radioactive waste, hole in ozone later, acid rain,
mass extinction, etc.

The future

4 600-9 000

The next 4 600 million years

What next? The world has changed so much in the last 4 600
million years who knows what will happen in the next.
In the short term- More pollution? More extinctions? Changes
in the atmosphere? Global warming? Rising sea levels and flooding
of the continental margins?
In the medium term—the extinction of human beings. What will
evolve to dominate the world then? The mammals had to wait for
the extinction of the dinosaurs before their sudden evolutionary
radiation. What is waiting for our extinction? Could it be the
turn of the insects? Or something totally unknown?
In the longer term—reorganisation of the continents. Greenhouse
and Icehouse Earth? And finally the sun becomes a red giant,
destroys the planets and dies.